Automated feedback control of an inhalation exposure system with discrete sampling intervals: testing, performance, and modeling.

The application of a proportional-integral-derivative (PID) control algorithm to an inhalation exposure system using a building automation system is described. Previous studies had utilized a control system in which concentration was monitored continuously and adjustments to the generator were made on a continuous basis. In this system, benzene vapor was generated into a chamber, and a gas chromatograph was used to measure the concentration in a chamber at discrete 30-min intervals. Thus only limited opportunities were available to sample and adjust the vapor generator flow rate. A series of tests were conducted in which the generator was operated without control, with control, with an additional load, and with nonoptimal settings. The results showed that the PID control loop could function effectively to restore a system back to the target set point, even with an additional load on the system. With nonoptimal control settings, the system showed oscillatory behavior. A model to simulate operation of the chamber was developed on a spreadsheet program. The model was accurate at simulating the various testing scenarios and useful for selecting the proper control settings. A PID feedback control system operating with a concentration monitoring system that sampled on a 30-min cycle was shown to produce exposures that were accurate in matching the target set point and maintaining a constant concentration.